20 Lovering and Bond on Magnetic Observations at Cambridge. 
straight lines or curves of the simplest curvature. From the details 
published in regard to the principle of observing it may be inferred 
how closely these curves will represent the actual magnetic changes 
for the day. It cannot be denied that disturbances may happen, of 
less amount than the minimum quantity of observation or at less 
periods than 5’, which will elude the vigilance and refinements of 
the present state of Magnetic science. It has been noticed on 
more than one occasion that the bar has been instantaneously 
checked in the midst of a vibration and forced to retrace its steps 
by along sweep in the opposite direction. The lines which are 
now drawn straight or in the most natural curve from one fixed 
point to another on the sheet might, if they were sensible of the 
shorter and more rapid magnetic impulses, change their curvature 
several times during the passage. 
Plates II. III. 1V. and V. represent the diurnal curves of mag- 
netic declination for the days given on the plates; and we are first 
to consider from an attention to them as well as to the figures which 
describe the other days at the end of the communication whether 
the fact of a regular cycle of variations in the declination during the 
24 hours is confirmed by these observations. The theory appears 
now to be well established, that the Elements of Terrestrial Magnet- 
ism are subject to daily, monthly, yearly, and secular perturbations 
similar to the periodical and secular variations which are known in 
astronomy. But in the astronomical problem, no derangement oc- 
curs whose cause is not looked for and generally found in the 
uniform operation of the simple law of gravity in its direct or 
reflected action upon the various members of the solar system. 
The singular fact of Encke’s comet, which experiences a delay 
which has been attributed to a resisting medium, may be regarded 
as a solitary exception to the general truth. The laws of motion 
